How were these 2 monitors set up and why did screen resolution end it?

Two monitors and one system. Three user accounts, the pc owner, me, and a guest.
Start > switch users > allowed user accounts from one to another. The guest account was never used.
I arrrive at the location for a few days. Two monitors, an Apple and a HP.
My user account had my own desktop from a previous visit. The only thing on my desktop was Google Chrome and Firefox.

When I arrived, there was a May and June calendar was seen on the HP monitor. The pc owner's account. The main monitor, the Apple had my desktop and it's two browser icons.

I wanted to use FF and IE. Start > Internet Explorer got me that browser.
Firefox was used too and on my use account, I'd have both browsers open.

I discovered that if I used my cursor to drag one of my browsers far enough left, the left portion would be seen on the HP monitor. I could, as an example, have half of the webpage on the HP monitor and the right half on the Apple monitor. Also, without dragging the webpages, I could move the cursor to the left and it would be seen on the HP monitor.

How did the pc owner have this setup so this was all possible?

Then the pc owner who was out of town and had access to his account on this same pc, wanted to adjust the screen resolution on his end for some reason, maybe on his laptop or other internet connected device.

After that my desktop was no longer on the Apple monitor but on the HP monitor.

No problems except:
When I opened FF, that browser opened on the Apple monitor and was very high resolution, very large.
The desktop on the HP monitor was normal size. When Chrome was opened it did so on the HP monitor. So, now even though the two browser icons were on the HP monitor desktop, FF would open on the Apple monitor and be very large and Chrome would open on the HP monitor.
I also tried to access the FF browser using my cursor. Once I moved the curser to the right to cause it to go to the Apple monitor where the FF browser was open, I could not move the cursor back to the HP monitor. I was stuck. The monotir with the proper resolution was the HP, but my cursor was stuck on the Apple monitor.
I had to re-boot, and abandon FF and just use the Chrome browser because the resolution was way to high for FF and FF would open on the other monitor as well as the fact that the cursor could no longer go back and forth between the two monitors.

How was all this setup by the pc owner and why did the attempt to adjust the resolution cause Chrome to open on one monitor but FF (from the same desktop location as the FF icon) would open on the Apple monitor?

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I use the same setup for work when I work on the server.This is in the screen resolution properties and you can use to extend your desktop to a second screen.How he managed to force a window to open into the screen he wanted: just open it,drag it into the desired screen then expand the window,it will stay in that screen,and whenever you open it,it will automatically get there.
If you got more questions,or my answer did not help,don't be afraid!

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The setup was intended. The Apple monitor in front of me was the "main" one.
That had my desktop for my user account on his pc.
For 4 days, everything was fine. The HP monitor off to the left only had calenders for May and June. I knew I could drag a browser to the HP monitor but did not.
So, that was my main question, how was it done.

The unknown question is what caused it to no longer work?
I got a phone call saying he needed to access his user account. He did and it looked like on my end he was changing the resoltion for some reason.
The FF browser I had open on the Apple monitor was so large I could not even find the cursor. Things were normal on the HP monitor as far as resolution. The only difference was now, instead of my desktop being on the Apple monitor, mine was not on the HP monitor.
I tried to finish what I was doing on that FF browser and had to move the cursor from the HP to the Apple to try to complete me work on FF. I finally got the cursor over to the Apple but could not go back to the HP with the cursor, where Chrome and my desktop was located. I was stuck, my desktop on the HP but the cursor on the Apple monitor and it would not move over to the HP. I did what we did not want to do and that was to reboot. That action resulted in my desktop and cursor being on the HP monitor.
I opened FF from the HP monitor but the FF browser did not open on the HP monitor it opened on the Apple.
Since what the pc owner did as far as resolution or other actions he took that I was not aware of, had caused FF to not open on the proper monitor, I just used Chrome the rest of the day.
I guess I'm not sure what he did to cause things not to work as before. What could have been done to cause the cursor to only be used on one monitor when before, you could go back and forth?

He changed the resolution so that he could see and use that Apple monitor on his local machine. What you could try now is to set the resolution probably to its native resolution. That should give you back your dual monitor setup. Hope this helps.